As Bader has shown in his
``Theory of Atoms in Molecules''
[13] one can define an atom via the
zero-flux surface of the electron density: Looking at the gradient vector
field, one finds that the all gradient vector trajectories terminate at only
one nucleus each. This allows to define an atomic basin as the space which is
crossed by the gradient vector trajectories starting at the corresponding
nucleus (cf. 4). The border of such a basin is the zero-flux surface.
This type of definition of an atom yields quantum
mechanically defined charges of atoms, which are more indicative of properties
than e.g. Mulliken charges.

A selection of charges and populations derived by different methods is given
in table 7.
For the silatrane studied here we find a very positive silicon atom and
corresponding negative nitrogen and oxygen atoms. This indicates a strong
ionic contribution to the bonding.

Table 7:
Atomic charges and populations. Columns marked with -- were not explicitly
calculated due to time limits.